The main objective of this project is to define by means of a permeability profile the selective permeability characteristics of the blood- brain barrier (cerebral capillary endothelium) in normal and abnormal conditions with particular emphasis on changes occurring with central nervous system tumors. The permeability profile represents the permeability coefficients of a number of substances which differ significantly in electrical charge, molecular weight, lipid soubility and metabolic importance. The permeability during normal and abnormal conditions will be measured by the indicator dilution technique, as well as simultaneous measurements of blood flow and capillary area. The solute extraction will be determined from the percent recovery of the injected dose of both the reference and the various test substances. In addition, measurements will be made simultaneously of blood gases; arterial, venous, and CSF pressures; EEG; and ECG. It is believed that alteations in selectiveity of this barrier, as evidenced by changes in the permeability characteristics of important ionic and non- ionic substaances, are fundamental to the understanding of the biology of brain tumors and other pathological conditions normally encountered in human neurologic disease.